I use and recommend the 25 watt Hydor Theo heater for all my tanks 5G and under. It should be fine in a 2.5G--I find them to be very accurate, and of the many I have owned, only one has ever stopped working--and I have had quite a few. I tend to give them away whenever a friend of mine gets a betta and doesn't realize it needs a heater, lol. It is best to order online since the average in-store price of these tends to be around $20-$27.

A 25 watt heater is designed for use in tanks that are 2 gallons in volume or larger, so you're good! Yes, it's totally fine if some of the cord is in the water. If your container is shallow, you can lay the heater down on its side to submerge it completely. If you use it vertically, just make sure that the water never goes below the water line indicated on the side of the heater.

D'oh, the product information on F&S gives the length of every one except the 25 watt. If I had to guess at it, it's probably about 5"--I don't have it in front of me at the moment since I'm at work, but it's definitely a bit shorter than the 50 watt, which is listed as 7".

I have the Mini Hydor heater, love it its like a little heating pad, they are preset and it keeps my 3 gal at 75 degrees which is what I want as I don't want higher for my bettas.

I did have the marineland one you talked about a few years ago, it was not very reliable and quit working all together 1 year later.

I hope this Hydor is a good one, time will tell. They can be buried if you want to or stuck to the back of the glass. I have mine stuck to the back of the glass as my background is black and you don't see it.